Biological treatment of wastewater for removal of dissolved organics is well known and is widely practiced in both municipal and industrial plants. This aerobic biological process is generally known as the “activated sludge” process in which micro-organisms consume the organic compounds through their growth. The process necessarily includes sedimentation of the micro-organisms or “biomass” to separate it from the water and complete the process of reducing Biological Oxygen Demand (BOD) and TSS (Total Suspended Solids) in the final effluent. The sedimentation step is typically done in a clarifier unit. Thus, the biological process is constrained by the need to produce biomass that has good settling properties. These conditions are especially difficult to maintain during intermittent periods of high organic loading and the appearance of contaminants that are toxic to the biomass.
Typically, this activated sludge treatment has a conversion ratio of organic materials to sludge of about 0.5 kg sludge/kg COD (chemical oxygen demand), thereby resulting in the generation of a considerable amount of excess sludge that must to be disposed of. The expense for the excess sludge treatment has been estimated at 40-60 percent of the total expense of wastewater treatment plant. Moreover, the conventional disposal method of landfilling may cause secondary pollution problems. Therefore, interest in methods to reduce the volume and mass of the excess sludge has been growing rapidly.
Membranes coupled with biological reactors for the treatment of wastewater are well known, but are not widely practiced. In these systems, ultrafiltration (UF), microfiltration (MF) or nanofiltration (NF) membranes replace sedimentation of biomass for solids-liquid separation. The membrane can be installed in the bioreactor tank or in an adjacent tank where the mixed liquor is continuously pumped from the bioreactor tank and back producing effluent with much lower total suspended solids (TSS), typically less than 5 mg/L, compared to 20 to 50 mg/L from a clarifier.
More importantly, MBRs (membrane biological reactors) de-couple the biological process from the need to settle the biomass, since the membrane sieves the biomass from the water. This allows operation of the biological process at conditions that would be untenable in a conventional system including: 1) high MLSS (bacteria loading) of 10-30 g/L, 2) extended sludge retention time, and 3) short hydraulic retention time. In a conventional system, such conditions could lead to sludge bulking and poor settleability.
The benefits of the MBR operation include low sludge production, complete solids removal from the effluent, effluent disinfection, combined COD, solids and nutrient removal in a single unit, high loading rate capability, no problems with sludge bulking, and small footprint. Disadvantages include aeration limitations, membrane fouling, and membrane costs.
Membrane fouling can be attributed to surface deposition of suspended or dissolved substances. The MBR membrane interfaces with the biomass which contains aggregates of bacteria or “flocs”, free bacteria, protozoan, and various dissolved microbial products (SMP). The term SMP has been adopted to define the organic compounds that are released into the bulk microbial mixed liquor from substrate metabolism (usually biomass growth) and biomass decay.
In operation, the colloidal solids and SMP have the potential of depositing on the surface of the membrane. Colloidal particles form layers on the surface of the membrane, called a “cake layer.” The MBR processes are designed to use rising course air bubbles to provide a turbulent crossflow velocity over the surface of the membrane. This helps to maintain the flux through the membrane, by reducing the build up of a cake layer at the membrane surface.
In addition to cake formation on the membrane, small particles can plug the membrane pores, a fouling condition that may not be reversible. Compared to a conventional activated sludge process, floc (particle) size is reportedly much smaller in typical MBR units. Since MBR membrane pore size varies from about 0.04 to about 0.4 micrometers, particles smaller than this can cause pore plugging. Pore plugging increases membrane resistance and decreases membrane flux.
In the activated sludge process, soluble organic compounds are converted into biomass and gaseous waste products by microorganisms. The amount of biomass produced, termed “sludge yield,” is related to the mass of carbon substrate in the influent per unit time. While biosolids production is lower in a MBR compared to ASP, removal or wasting of biosolids is necessary to maintain the overall performance of the system. Typical MBR biosolids concentration, expressed as mixed liquor suspended solids, is 1-2%, so that water is 98-99% of the wasted sludge. To reduce costs, therefore, it is preferable to dewater the sludge and decrease the volume prior to disposal. Typically, filtrate from the dewatering process is recycled back to the MBR.
Synthetic, water soluble polymers are commonly used to condition sludge prior to introduction into a dewatering device, such as a belt filter press or centrifuge. These polymers partition between the solids and the water phase, often leaving 5-50 ppm or more polymer product in the water (filtrate). These polymers have the potential to foul membranes similar to SMP membrane fouling. Because of the potential fouling effect of high molecular weight polymers on membranes, the use of such polymers in membrane systems has been patently avoided, including in ancillary dewatering processes where only filtrate is returned to the MBR.
Accordingly, there is an ongoing need for methods of using dewatering polymers in a manner that minimizes the amount of residual polymer in the dewatering process filtrate water in order to minimize the potential of membrane fouling while simultaneously achieving efficient dewatering performance in order to minimize sludge disposal costs.